A newly released memo by a wartime Japanese official provides what a historian says is the first look at the thinking of Emperor Hirohito and Prime Minister Hideki Tojo on the eve of the Japanese attack on Pearl Harbor that thrust the U.S. into World War II.

While far from conclusive, the five-page document lends credence to the view that Hirohito bears at least some responsibility for starting the war.

At 8:30 p.m. in Tokyo, just hours before the attack, Tojo summoned two top aides for a countdown to war briefing. One of them, Vice Interior Minister Michio Yuzawa, wrote an account three hours after the meeting was over.

“The emperor seemed at ease and unshakable once he had made a decision,” he quoted Tojo as saying.